The mind should guard the heart in the time of prayer and always stay inside it.
From there, from the depths of the heart, it should then lift up the prayers to God.
For once it tries inside the heart and tastes and is soothed – as the Lord is good! – then the mind will never want to leave the place of the heart.
It will there repeat the words of Peter the apostle: “It is wonderful for us to be here!” (Matt 17:4).
Then it will always wish to look inside the heart, remaining there and pushing aside and expelling all the concepts which are planted by the devil.
To those who have not realised this work of salvation and remain unaware of it, this will most of the times seem very hard and unpleasant.
But those who have tasted its sweetness and enjoyed the pleasure inside the depths of their hearts, they all cry together with Paul: “What could ever come between us and the love of God?” (Rom 8:38-39).
Our holy fathers have listened the Lord who said that from the heart come evil intentions, murder, adultery, fornication, theft, perjury, slander (Matt 15:19-20) and how these are the things that make a man unclean.
Further, they have listened to the part of the gospel where we are ordered to clean the inside of cup and dish first so that the outside may become clean as well (Matt 23:26).
They therefore left aside any other spiritual work and concentrated exclusively on guarding the heart, being confident that through this they would easily achieve all other virtues, whilst without it no virtue can be preserved.
This practice was called by some fathers “serenity of the heart”, whilst others named it “attention”, others “sobriety” and “detainment”, others “examination of the thoughts” and “guarding of the mind”.
[…] It is for this that the Ecclesiastes says: Rejoice, O young man, in thy youth; and let thy heart cheer thee in the days of thy youth, and walk in the ways of thy heart blameless and clear, and prevent your heart from thoughts (Eccl. 11:9 [LXX]).
The same is said in the Proverbs: if the devil makes an assault on you, do not let him enter your place (Eccl. 10:4 [LXX]) where “the place” means the heart.
The Lord Himself tells us in the Gospel that we must not worry (Lk 12:29) – in other words, not to scatter our minds here and there.
Again, in a different passage He says: Happy are those poor in spirit (Matt 5:3), meaning that happy are those who never acquired any concern of this world in their hearts and are free from all earthly thoughts.
Symeon the New Theologian (949–1022 AD): [traditional attribution] The Three Ways of Attention and Prayer Translated from Greek by Demetrios S. Skagias @ Myriobiblios.